生物分子NMR中不稳定质子的异源转移：交叉极化，再访
Heteronuclear transfers from labile protons in biomolecular NMR: Cross Polarization, revisited
摘要：
基于INEPT和HMQC的脉冲序列被广泛用于在异核之间转移偏振，特别是在生物分子光谱学中：它们易于设置并且涉及低功率沉积。尽管如此，这些短脉冲极化转移方案受到快速溶剂化学交换的挑战。改善这些异质性转移的替代方案是J驱动交叉极化（J-CP），其通过在Hartmann-Hahn条件下自旋锁定耦合自旋来转移极化。J-CP提供了一定的免疫力，对化学交换和其他T2类松弛效应，在这里深入研究的行为，刘维空间数值和分析推导描述的传输效率。虽然优于基于INEPT的传输，但快速交换也可能会减慢这些J-CP传输，从而损害其效率。因此，本研究探讨了在快速溶剂化学交换存在下，重复投影操作改善1H-> 15 N和1H-> 15 N-> 13 C J-CP转移的潜力。发现虽然重复的J-CP相对于延长的CP提供很少的1H-> 15 N转移优势，但是保持水和不稳定质子有效自旋锁定的多个接触可以在化学交换的存在下改善1H-> 15 N-> 13 C转移。随后的循环，级联交叉极化（L-CCP）通过依赖13 C更长的寿命来补偿单个J-CP损失，从而产生一种算法冷却，可以为15 N以及羰基和α 13 Cs提供高极化。这可以促进某些实验，如对涉及不稳定的化学交换质子的内在无序蛋白质的三重共振实验所证明的。
英文摘要：
INEPT- and HMQC-based pulse sequences are widely used to transfer polarization between heteronuclei, particularly in biomolecular spectroscopy: they are easy to setup and involve low power deposition. Still, these short-pulse polarization transfers schemes are challenged by fast solvent chemical exchange. An alternative to improve these heteronuclear transfers is J-driven cross polarization (J-CP), which transfers polarization by spin-locking the coupled spins under Hartmann-Hahn conditions. J-CP provides certain immunity against chemical exchange and other T2-like relaxation effects, a behavior that is here examined in depth by both Liouville-space numerical and analytical derivations describing the transfer efficiency. While superior to INEPT-based transfers, fast exchange may also slow down these J-CP transfers, hurting their efficiency. This study therefore explores the potential of repeated projective operations to improve 1H->15N and 1H->15N->13C J-CP transfers in the presence of fast solvent chemical exchanges. It is found that while repeating J-CP provides little 1H->15N transfer advantages over a prolonged CP, multiple contacts that keep both the water and the labile protons effectively spin-locked can improve 1H->15N->13C transfers in the presence of chemical exchange. The ensuing Looped, Concatenated Cross Polarization (L-CCP) compensates for single J-CP losses by relying on the 13C longer lifetimes, leading to a kind of algorithmic cooling that can provide high polarization for the 15N as well as carbonyl and alpha 13Cs. This can facilitate certain experiments, as demonstrated with triple resonance experiments on intrinsically disordered proteins involving labile, chemically exchanging protons.
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